This invention relates to a method for mapping drillable areas in an on-shore or off-shore oil field, without the risk of encountering anomalous zones, such as surface gases.
We know that surface gases are located between the surface of the ground and a depth at which the well bore has not yet been equipped with devices that make it possible to stop a potential gas inlet. These gases are a potential and very serious danger when drilling for oil and their location and significance must be evaluated prior to installing a well bore.
Until now, the detection of surface gases was carried out by using the exploration seismic reflection method called two-dimensional (2D). This method consists of gathering seismic traces resulting from the reflection of acoustic waves by the subsoil using an acquisition apparatus that consists of at least one acoustic wave emitting device and one line of acoustic receivers, and that is moved along the surface of the ground (land shooting) or the surface of the sea (marine shooting).
In particular, in 2D marine seismic exploration, the data acquisition equipment consists of a boat equipped with an acoustic impulse emitting source that tows a line or streamer on which are assembled a plurality of equally spaced sensors designed to receive the acoustic waves reflected by the various layers of the subsoil and where each one delivers a signal that represents the amplitude of the waves received in relation to time, this signal is recorded in order to provide a record called seismic trace. The length of the streamer may vary from approximately 500 m to approximately 6000 meters and the sensors fitted on the streamer are spaced from a few meters to several tens of meters apart, for example from around 12.5 meters to 50 meters. The marine area to be studied in the 2D seismic exploration method can range from several hundreds of square kilometers and can, for example, have a surface of 200 km.times.200 km. The boat covers this area following straight parallel paths, several kilometers apart, for example 5 to 6 km apart. During its movement, the seismic source emits impulses at regular time intervals, for example every 5 seconds. After treating the seismic recordings, for each straight path of the boat we obtain a 2D seismic section consisting of a plurality of vertical traces. This section represents the vertical cut of the subsoil in a system of coordinates X (direction of the boat's movement) and T (depth expressed in time).
In the case of shallow sands saturated with gas, the seismic energy that is reflected can take on significant values that are translated on the recorded seismic section by peaks of high amplitude (bright spots).
In order to reconstruct the image of the subsoil of the area being studied, we must put the various recorded seismic sections side by side and imagine interpolations between the bright spots of such sections. However, these interpolations are lengthy operations that are somewhat subjective. Indeed, it is not unusual for two interpreters to carry out the same interpolations in different ways and come up with maps with different risks.
We also know the three dimensional (3D) seismic acquisition method. This method uses an acquisition device that consists of at least one source that emits acoustic waves and a plurality of lines of acoustic sensors that is moved along the surface of the ground (3D land seismic acquisition) or the surface of the sea (3D marine seismic acquisition). In using this method in marine seismic acquisition, a boat equipped with at least one source of emitting acoustic waves tows several lines or streamers arranged parallel to each other, whose number can reach 8, where each line has a plurality of acoustic wave sensors. The lines are shorter than in the previous 2D seismic acquisition, are separated from each other for example by approximately 50 m and carry sensors that are equally spaced, for example every 25 m. The width of the area covered by the boat increases with the number of lines of sensors and is, for example, in the range of 400 m in the case mentioned above.
With each recorded trace, we associate a spatial position defined using the coordinates of the associated emitting source and sensor at the time of the shot that will produce this trace.
From the recorded seismic traces during the 3D seismic acquisition, we can achieve an image of the subsoil in three dimensions in an (X, Y, T) axial system in the form called "3D seismic cube" (3D seismic section). For this, we attribute to each square (bin) of a set of squares that makes up a regular spatial grid in a plane (X, Y) representative of the acquisition plane of the seismic traces, a central trace obtained from the recorded traces using the well known multiple coverage technique, and whose spatial positions are located inside the square in question, this central trace is assigned to the center of such square following the time-dependent axis T. The seismic block is sampled in elementary parallelepiped fictive cells each centered on a bin and whose dimensions following X and Y correspond to those of such bin and whose width according to T corresponds to the length of the time sample chosen to sample the central seismic trace, each cell contains a sample of seismic trace.
In this way, we obtain a continuous sampling of the subsoil. Any cell C.sub.ijk of the seismic block being sampled is therefore perfectly defined by the coordinates (X.sub.i, Y.sub.j, T.sub.k) of its center, or in other words, of the central seismic trace sample that it contains, where X.sub.i and Y.sub.j are the coordinates of the B.sub.ij bin associated with the cell and T.sub.k is the time-dependent coordinate of the sample k of the central seismic trace TR.sub.ij of bin B.sub.ij, and by the amplitude A.sub.ijk of such sample. The bins that cross-rule the 3D seismic block's (X, Y) plane are preferably rectangular and correspond with rectangles on the ground with, for example, a length of 50 m (distance between the lines) and a width of 25 m (distance between two successive sensors on the line).
The useful information gathered during a shot is concentrated inside a conic volume centered on the space position of the recording (trace). The conic volumes that correspond to two successive shots emitted during the movement of the boat either do not cover each other or only partially cover each other for the surface layers of the subsoil. Therefore, the 3D seismic acquisition method with the parameters used for oil prospecting, whose objective is recognition in depth, is not favorable when emphasizing objects on the surface and in particular surface gases.
Thanks to the EP-A-0 562 687 patent we know of a method for locating hydrocarbon reserves using a structural interpretation of the data. In this method, from hypotheses on the presence of hydrocarbons in a region of interest of the subsoil:
(i) we select areas where the probability that they contain hydrocarbon reserves is greater than a predetermined probability, PA1 (ii) we define a structural closure for each of such selected areas from the structural data of the region of interest where each structural closure contains a surface capable of containing a hydrocarbon reserve and having a constant depth perimeter substantially equal to the deepest point of the area, PA1 (iii) we determine a measure of the geometric similarity between each area and the structural closure associated thereto, and PA1 (iv) we select each of the areas for which this measure exceeds a predetermined value.
However, it is obvious that this method has no relation to the method for mapping consistent with the invention and that the latter does not provide any element susceptible of suggesting in the least part this method.
We also know from the U.S. Pat. No. 5,153,858 patent, that represents the closest state of the technique, a method of automatic horizon pinpointing in a 3D block of seismic cells sampled in depth. In this method, each seismic trace of the 3D block is converted into a binary trace consisting of a sequence of values "1" and values "0", where value "1" is assigned to each sample of the seismic trace for which a horizon exists at the depth defined by this sample, and the value "0" to each sample that does not answer to this condition. In this manner, we transform the 3D block of seismic traces into a corresponding 3D block of binary traces, that is stored in the memory of a computer and is used for the search of automatic horizons. In order to do this, we chose a sample "1" of a binary trace of the 3D block of binary traces, that is located on the horizon to be pinpointed and, operating step by step on the adjacent binary traces, we carry out an automatic sweeping of the binary 3D block to find all samples with a value of "1" that correspond to the initial sample "1". From these depths associated to the selected samples, we create a representation of the sought horizon.
The method consistent with the patent has as starting point a 3D block of seismic traces, but the treatment of this block of seismic data by applying such method is different from the treatment consistent with the invention and leads to a completely different result.